Windows 7 bought with it the increased popularity of 64-bit operating systems on the desktop, replacing the older, memory-constrained 32-bit platform. But just because your system can handle a 64-bit OS doesn't mean that you're ready for 64-bit Office 2010.
Over on the Office 2010 Technology Guarantee FAQ page, Microsoft "strongly recommends" that users install the 32-bit version of the new Office suite:
Will a 64-bit version of the Office 2010 product be available?
Yes, 64-bit Office 2010 product upgrades will be available. However we strongly recommend most users install 32-bit version of Office 2010 on both 32 and 64-bit Operating Systems because currently many common add-ins for Office will not function in the 64-bit edition. The 64-bit installation of Microsoft Office 2010 products will be available for users who commonly use very large documents or data set and need Excel 2010 programs to access greater than 2GB of memory. There may be technical issues with the 64-bit version and in order to install a 64-bit version of Office 2010 product users must have a 64-bit supported operating system on their PC.
Add-ins are likely to be Office 2010 64-bit biggest Achilles' heel, given the fact that there are very few 64-bit add-ons available, even from Microsoft. Add-ons for elements such as antivirus programs are likely too be a major sticking point.
If you use add-ons to augment your Office suite, you're likely to be tied to 32-bit for many months following the release of the Office 2010.
(via PC Pro)